The present invention relates to piercing tubes, and more particularly to piercing liquid-filled metal tubes.
Two conventional methods of cold forming metal tubes to create structural members, for example, for the automotive industry, are hydroforming and liquid impact forming. In a typical hydroforming process, a tube is partially deformed by stamping it in a die. Then, internal hydraulic pressure exceeding the yield strength of the tube wall is applied to force the tube to expand and conform to the die cavityxe2x80x94much like blowing up a balloon. In the liquid impact forming process, a tube is filled with liquid at atmospheric pressure and sealed. The tube is stamped in a die cavity. During stamping, the liquid resists the compressive forces generated and forces the tube walls to correspond to the configuration of the die cavity.
After either the hydroforming process or the liquid impact forming process, holes are typically pierced or punched into the structural tube, for example, to provide points of attachment. Typically, punching holes deforms the metal surrounding the hole. The material surrounding the holes typically is crater-shaped around the punched hole. As depicted in FIG. 1 the tube 4 is pierced by a punch 110 to form a slug 112 and define a hole 114. Because tube wall 10 is not backed up during the punch process, tube wall 10 deforms the material 116 in the area surrounding the hole 114 and the punch 110.
In one attempt to reduce deformation surrounding the hole, the tube is xe2x80x9cpre-bulgedxe2x80x9d in the area where the hole is to be pierced. The outwardly bulged material is depressed and substantially flattened during punching of the hole. This technique is disclosed in U.S. Pat. No. 5,813,266 issued Sep. 29, 1988, to Ash. As a result, the pierced tube has less deformation surrounding the hole than a comparative tube pierced without an outwardly bulged area. While reducing deformation, this process requires the additional formation of a bulge in the metal tube around punched holes, thereby adding another step to the tube forming process and increasing the cost of stamping.
The aforementioned problems are overcome in the present invention wherein a liquid-filled tube is pierced with a tapered punch during stamping to form a hole with minimal deformation of material surrounding the hole. More specifically, a preferred process of liquid impact forming or hydroforming, and piercing a tube includes the following steps. First, a sealed liquid-filled tube is positioned within a die cavity. Second, the tube is stamped causing the internal pressure of the liquid in the tube to be boosted due to the compressive force of the stamping so that the exterior of the tube conforms to the interior of the die cavity. Third, a tapered punch in the die cavity pierces through a tube wall. Due to the elevated internal pressure of the tube, the deformed material adjacent the punched hole is pushed back against the die wall. The tapered punch permits the free movement of the material to its pre-punch position. Finally, the tapered punch is retracted from the die cavity. During retraction, the relatively large end of the tapered punch catches on the material adjacent the hole pulls the material back against the die wall.
The present invention provides an efficient and economical process to pierce stamped tubes and create clean, well-defined holes in the tubes without the need for additional machining.